It is the objective of this invention to provide an economical solution to the high oxygen transmission biaxially oriented polypropylene (BOPP) film.
U.S. Pat. No. 6,232,402 (DeMeuse '402), discloses BOPP film comprising a mixture of 75–92% by weight isotactic polypropylene, 5–15% by weight low density polyethylene, and 3–10% by weight of an olefin heteropolymer containing polypropylene and at least one other 2–4 carbon alpha olefin. The invention cited examples claim oxygen transmission values in the range of between 206.0–285.6 cc/100 in2/day. These oxygen transmission values are quite low as compared to the present invention. In addition, the presence of low density polyethylene and olefin heteropolymer in the formulation decreases the thermal properties of the BOPP film that are necessary during sealing.
U.S. Pat. No. 6,410,136 (DeMeuse '136), discloses a biaxially oriented film comprised of a mixture of 10%–60% by weight of isotactic polypropylene, 10%–25% by weight of low density polyethylene and about 15%–80% by weight of an olefin heteropolymer containing polypropylene and at least one other alpha olefin of 2 to 4 carbon atoms. The invention cited examples claim oxygen transmission values in the range of between 323.3–589.6 cc/100 in2/day. These oxygen transmission values are in the range of the present invention, however the high levels of heteropolymer and low density polyethylene decreases the thermal properties of the film that are necessary during sealing.
U.S. Pat. No. 6,395,071 (Niijima), et al discloses a breathing film which contains a block copolymer composition comprising crystalline polypropylene and propylene alpha olefin random copolymer having 45–85% by weight of propylene unit based on the weight of copolymer. The film thickness is between 10 and 100 microns and having an antibacterial agent coated thereon or included therein. The invention cited examples claim oxygen transmission values in the range of between 781.3–800.8 cc/100 in2/day. These oxygen transmission values are very good; however the tensile modulus values are in the range of from 122,000–126,000 psi, which is very low for the application.
U.S. Pat. No. 6,485,817 (Demeuse '817) discloses a transparent composite oriented film comprising a first layer and at least a second layer, said first layer consisting essentially of an ethylene/propylene copolymer including less than 10% by weight ethylene, said at least second skin layer being a high tensile modulus polypropylene. The invention examples indicate oxygen transmission values in the range of 215.2–363.7 cc/100 in2/day. These oxygen transmission values are lower than the present invention.
U.S. Pat. No. 6,348,271 (Nakata) discloses a multilayer film having a thickness of 10–100 microns, wherein the film comprises at least one layer obtained by processing and drawing a block copolymer composition in at least monoaxial direction and the block copolymer composition comprises 93–30% by weight of a propylene polymer component and 7–70% by weight of an ethylene/propylene copolymer. This polymer is essentially a block copolymer where the ethylene/propylene copolymer content is tailored to balance the film transparency, tensile properties and oxygen transmission of the film. The example cited by this patent indicates oxygen transmission values in the range of 244–762 cc/100 in2/day. However, as the oxygen transmission increases, the tensile properties are also reducing dramatically.
These prior art structures use either ethylene/propylene copolymers, polypropylene block copolymers including ethylene/propylene rubber, or three component blends with low density polyethylene. These methods of increasing oxygen transmission are only partially successful because each type sacrifices one or more desired film attributes in order to improve the oxygen transmission.
It is the object of this invention not only to increase the oxygen transmission of the film, but also to maximize gloss, tensile strength, and thermal resistance while minimizing haze. This BOPP film with balanced properties performance satisfies the total film requirements where the prior art structures do not fully satisfy all of these criteria.